| Abstract | This research examined the photocatalytic decolorization of synthetic and real dyeing
wastewater. Synthetic wastewater was a solution containing 15-30 mg/L of reactive black 5, a
reactive azo dye. Ti02 as well as CdS photocatalysis were carried out in the synthetic
wastewater studies whereas only Ti02 photocatalysis was applied to decolorize real
wastewater collected from cotton dyeing, printing and finishing process. An aerated
semiconductor slurry was illuminated in a batch-lab-scale reactor.
Effects of the influencing factors:-initial pH, catalyst dosage, light intensity, V/IA
ratio, solution temperature and initial dye concentration-were explored. Even under
anaerobic conditions, the decolorization could be observed in both photocatalysis. External
aeration played less important role in CdS than in Ti02 photocatalysis. The minimum first
order rate constant (k1st) of RB5-Ti02 photocatalysis was recorded at pH 7 and k1st increased
in acidic as well as in alkaline range while for CdS photocatalysis, the maximum rate constant
was found at pH 4. This different pattern reflected the difference in reaction mechanisms of
these two reactions. Solution pH affects the radical formation, the position band shift and the
electrostatic adsorption of dye onto catalyst surface, depending on molecular structures of
dye, investigated by FTIR and amphoteric behavior of catalyst surface.
The main mechanism of Ti02 photocatalysis was the oxidation by radicals, especially
hydroxyl radical. The enhancement in alkaline region was owing to the increase of hydroxyl
ions affecting the hydroxyl radical formation whereas the enhancement in acid region was due
to increase of adsorption capacity. Direct oxidation by valence band holes was expected to be
the main mechanism of CdS photocatalysis. Thus, the increase in hydroxide ions did not
enhance the reaction rate (pH 4 to 11). The decrease of reaction rate with increase in pH may
be attributed by the decrease of RBS adsorbed onto CdS surface and by the increase of Cd
deposition and Cd(OH)2 precipitation on CdS surface.
The first order rate constants of both Ti02 and CdS photocatalysis increased with
increase in the amount of catalyst (0-1.0 g/L for Ti02 and 0-1.5 g/L for CdS) and remained
almost constant above a certain level. The k1st values of both reactions increased in a nonlinear
pattern with increased light intensity. Ti02 as well as CdS photocatalysis followed the
Arrhenius equation in the range of 303-333 °K. The activation energy of both reactions was of
the same order of magnitude. The reaction rate of both reactions were significantly magnified
as V/IA ratio decreased continuously from 7.62 to 1.27 mL/cm2. A linear relationship between
k1st and the initial concentration (15-75 mg/L) was observed for Ti02 photocatalysis, and in
contrast, the non-linear relationship was found for CdS photocatalysis. Although lower
percentage of color removal was associated with higher initial dye concentration, the total
amount of dye removal increased. Energy per volume of Ti02 and CdS photocatalysis were 38.S and 47.7 kWh/m3, respectively and EE/O of both systems were 46.7 and S7.9 kWh/order/m3, respectively.
CdS photocatalysis followed both Langmuir Hinshelwood and Eley-Rideal kinetics, (at
pH 7) while Ti02 photocatalysis of RBS could not be explained by two mentioned kinetics.
The RBS photocatalytic degradation pathway identified by UV-visible absorption and FTIR
studies was proposed that the azo linkages were the sites of attachment and then, the RBS
molecule was cleaved at the azo linkages and phenyl-N bonds. The complete mineralization of
RBS did not occur during the 180 min treatment period.
For real wastewater photocatalysis, the obvious enhancement of photocatalytic
decolorization in alkaline region was found owing to the increase of hydroxyl ions and the
repulsion between catalyst surface charges and inhibitor ions at pH > 6.3 (IP of Ti02).
Photocatalysis was also reinforced by the increase of temperature. The original pH and
temperature of wastewater at the collected station were 9.8-11.2 and 48-S2 °C respectively;
thus, pH and temperature adjustments before photocatalysis were not necessary. The optimum
V/IA was 1.27 mL/cm2 (solution depth = 1.2 cm). The effect of Ti02 dosage and UV light
intensity on real wastewater photocatalysis was similar to RBS wastewater. The optimum Ti02 dosage was 2 g/L and the optimum UV light intensity was 14.2 mW/cm2. Energy per
volume was 157.5 kWh/m3 and EE/O was 191.2 kWh/order/m3. The potential of
decolorization by Ti02-solar light was demonstrated. With constant light intensity, treatment efficiency increased when V/IA ratio was decreased. During the 3-hr photocatalysis, the color and the COD in the wastewater continuously decreased whereas the BOD increased. Indicated
by the BOD:COD ratio, the biodegradability of treated wastewater was higher than the raw one. Under the optimum operating conditions, time to achieve 300 ADMI for synthetic and real wastewater were 18 and 63 min, respectively.
The numbers of Ti02 reuse times in synthetic and real wastewater photocatalysis were
9 and 7, respectively. The catalyst activity of reused CdS was significantly lower than the new
one due to Cd deposition and Cd(OH)2 precipitation on CdS surface. Results of Microtox®
test showed that synthetic as well as real wastewater were detoxified by Ti02 photocatalysis.
In contrast, toxicity of RBS wastewater treated by CdS photocatalysis significantly increased
due to the CdS photocorrosion. In conclusion, this research showed the potential of Ti02
photocatalytic decolorization of dyeing wastewater without any increase of toxicity in
wastewater. |
